Abstract
Abstract. Coastal waters globally are increasingly impacted due to the anthropogenic loading of nitrogen (N) from the watershed. To assess dominant sources contributing to the eutrophication of the Little Narragansett Bay estuary in New England, we carried out an annual study of N loading from the Pawcatuck River. We conducted weekly monitoring of nutrients and nitrate (NO3-) isotope ratios (15N / 14N, 18O / 16O, and 17O / 16O) at the mouth of the river and from the larger of two wastewater treatment facilities (WWTFs) along the estuary, as well as seasonal along-river surveys. Our observations reveal a direct relationship between N loading and the magnitude of river discharge and a consequent seasonality to N loading into the estuary – rendering loading from the WWTFs and from an industrial site more important at lower river flows during warmer months, comprising ∼ 23 % and ∼ 18 % of N loading, respectively. Riverine nutrients derived predominantly from deeper groundwater and the industrial point source upriver in summer and from shallower groundwater and surface flow during colder months – wherein NO3- associated with deeper groundwater had higher 15N / 14N ratios than shallower groundwater. Corresponding NO3- 18O / 16O ratios were lower during the warm season, due to increased biological cycling in-river. Uncycled atmospheric NO3-, detected from its unique mass-independent NO3- 17O / 16O vs. 18O / 16O fractionation, accounted for < 3 % of riverine NO3-, even at elevated discharge. Along-river, NO3- 15N / 14N ratios showed a correspondence to regional land use, increasing from agricultural and forested catchments to the more urbanized watershed downriver. The evolution of 18O / 16O isotope ratios along-river conformed to the notion of nutrient spiraling, reflecting the input of NO3- from the catchment and from in-river nitrification and its coincident removal by biological consumption. These findings stress the importance of considering seasonality of riverine N sources and loading to mitigate eutrophication in receiving estuaries. Our study further advances a conceptual framework that reconciles with the current theory of riverine nutrient cycling, from which to robustly interpret NO3- isotope ratios to constrain cycling and source partitioning in river systems.
Highlights
Human activities have resulted in a substantial increase in the delivery of nutrients from terrestrial to aquatic and marine systems (Gruber and Galloway, 2008)
The concentration of NO−3 measured in samples collected weekly at the Stillman Bridge was lowest in winter and highest in the summer months, ranging from to 9.7 μM to as high as 73.5 μM, with a median value of 30.4 μM (Fig. 2a)
Values of δ18ONO3 followed a contrasting trend, being lower during the summer months and increasing in winter months, with values ranging from 1.6 ‰ to 6.8 ‰, notwithstanding a single outlying value of 8.1 ‰ (Fig. 2e)
Summary
Human activities have resulted in a substantial increase in the delivery of nutrients from terrestrial to aquatic and marine systems (Gruber and Galloway, 2008). In marine systems, increased loading of reactive nitrogen (N) has resulted in coastal eutrophication, engendering the loss of valuable nearshore habitat such as seagrass beds and oyster reefs, depletion of dissolved oxygen (creating so-called “dead zones”), and increased frequency and severity of algal blooms – including toxic brown and red tides causing fish kills (Heisler et al, 2008). Rollinson et al.: Seasonality of nitrogen sources, cycling, and loading (WWTFs), septic systems, industrial discharge, fertilizer applied to turf and agricultural lands, and atmospheric sources from industry and fossil fuel use (Valiela et al, 1997; McClelland et al, 2003; Latimer and Charpentier, 2010). The pervasive degradation of coastal marine ecosystems is alarming and of significant concern to coastal communities worldwide
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